Q(z) model building using walkaway VSP data

Abstract

Advanced seismic processing applications that use inverse [Formula: see text]-filtering for enhanced seismic resolution and true amplitude processing require reliable [Formula: see text] earth models. While interval [Formula: see text] may be accurately estimated by zero offset VSP surveys, quality data are obtained over relatively limited depth intervals, and shallower well sections pose special problems because of hole size, multiple casings, and lack of cementation. To obtain a more complete [Formula: see text] model, we have developed a theoretical method for combining zero offset VSP [Formula: see text] measurements with either effective-[Formula: see text] measurements from walkaway VSP surveys or surface seismic common midpoint (CMP) gathers. Using a simple crustal model of isotropic and homogenous flat layers, we derive an analytical result for the effective-[Formula: see text] measured from walkaway VSP surveys or from CMP gathers. We have found that effective-[Formula: see text] approaches a weighted harmonic mean ⟨[Formula: see text] of the overburden interval [Formula: see text]-values for short source-receiver offsets. This new method allows [Formula: see text] models to be constructed from the surface down to the total depth of the well, or even deeper, by combining offset-dependent effective-[Formula: see text] measurements with depth-dependent interval [Formula: see text] estimates from zero-offset VSP. The technique is also a simple method of compensating surface seismic gathers for anelastic [Formula: see text] absorption in an offset-dependent and time-variant way. We tested our new method using both synthetic data and walkaway and zero-offset VSP field data from the North Sea.